Microlithography is used for producing microstructured components, such as integrated circuits or LCDs, for example. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination device and a projection lens. The image of a mask (reticle) illuminated by the illumination device is in this case projected by the projection lens onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
The operating wavelength of the system, abbreviated to “operating wavelength” here and in the following, is the wavelength of the light at which the illuminated mask is optically imaged onto the substrate. If the light has a notable bandwidth, the operating wavelength is understood to mean the central wavelength.
In projection lenses designed for the extreme ultraviolet (EUV) range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, mirrors are used as optical components for the imaging process. Typical projection lenses designed for EUV, as are known e.g. from U.S. Pat. No. 7,538,856 B2, may, for example, have an image-side numerical aperture (NA) in the range of NA=0.2 to 0.3 and image an (e.g. ring-segment-shaped) object field (also referred to as “scanner slit”) into the image plane or wafer plane. A problem arising in practice in the case of approaches for increasing the image-side numerical aperture (NA) is that, in many respects, there are limits to increasing the size of the mirror surfaces required to accommodate this increase in image-side NA:
Firstly, it becomes increasingly difficult with increasing dimensions of the mirrors to reduce long-wave surface errors, in particular, to values below the required thresholds, with the larger mirror surfaces requiring, inter alia, stronger aspheres. Moreover, larger processing machines are required for manufacturing purposes in the case of increasing dimensions of the mirrors, and stricter requirements are placed upon the employed processing tools (such as e.g. grinding, lapping and polishing machines, interferometers, cleaning and coating installations). Furthermore, heavier mirror bases need to be used for the purposes of manufacturing larger mirrors, which mirror bases are only barely able to be assembled above a certain limit or bending beyond an acceptable measure due to gravity. Moreover, as the mirror dimensions increase, so does the operating outlay required to manufacture a mirror anew if even only a comparatively small portion (“scratch”) becomes damaged on the mirror.
In order to accommodate the problems associated with the increasing mirror dimensions mentioned above, it is known to fashion one or more mirrors in the imaging beam path of the projection lens in a segmented manner, i.e. to replace each monolithic mirror by a segmented mirror which is composed of a plurality of separate mirror segments.
When using such segmented mirrors in a microlithographic projection exposure apparatus, it is very important to avoid bothersome wavefront jumps between the partial beam paths emanating from the individual mirror segments. A further problem encountered in this situation is that the microlithographic imaging process for generating a sharp image requires not only the correct geometric-optical superposition of the images generated by the individual partial beam paths in the image plane of the projection lens but also the superposition thereof with the correct phase, i.e. it requires the individual mirror segments of the segmented mirror to have a common phase angle.
A common phase angle of the mirror segments of a segmented mirror should not only be set prior to first activating the system (e.g. after the system has been transported) but it should also be re-established after replacing one or more mirror segments of the segmented mirror, for example.
In respect of the prior art, reference is made in a purely exemplary manner to WO 2012/059537 A1, US 2012/0300183 A1, US 2011/0001947 A1, WO 03/093903 A2 and U.S. Pat. No. 8,228,485 B2.